Connecting rods were originally made by casting or forging separate attachable cap and body portions. These parts were usually made of medium carbon wrought steel and were separately machined at both joining and thrust faces; they were then separately drilled with holes to accept fasteners.
A first evolutionary step was to cast or forge the connecting rod as a single steel piece, followed by drilling of holes to accept fasteners. The single piece was sawn to obtain cap and body portions which were separately rough-machined at the thrust and contacting surfaces; the two portions were then bolted together for finish-machining. The separate steps of machining and sawing were not only cumbersome and expensive, but they did also not ensure a perfectly matched cap and body under the operating forces. Under some operating conditions, inherent diametrical fastener clearance permitted slight shifting of the cap and body portions, which in turn affected the bearing life.
As the next step in this evolutionary change, a single-piece connection rod was split or cracked into cap and body portions in the hope of providing non-sliding surfaces, where the cap and the body portions are bolted together. It was hoped that if the surfaces were properly remated, the remate would prevent any microshifting and assure accurate operating alignment. To split the single piece into two, it was initially struck on one side with a sharp blow. This met the requirements with limited success because of the difficulty in controlling the crack propagation and the risk of plastic deformation and possible damage to the blank.
Further developments have been various methods of fast tearing parts from each other along a well defined notch, which can be machined, forged, laser-cut, etc. Also these methods may introduce small plastic deformations in the fracture surfaces, which in turn can cause difficult mating of the split parts.
Common features of all commercial production of connecting rods are a final machining step to ensure the position, straightness and roundness of the bores in the big and small ends of the rod to prepare for the assembly of the bearings or bushings. To split the single piece into two, it was initially struck on one side with a sharp blow. This met with little success because of the uncontrollability of the cracking plane and possible damage to the connecting rod.
More recent and improved methods of splitting are disclosed in e.g. U.S. Pat. Nos. 2,553,935, 3,993,054, and 4,923,674 (Krebsoge).